Acta Crystallographica Section C

Crystal Structure Communications

Volume 64, Part 9 (September 2008)

inorganic compounds

Acta Cryst. (2008). C64, i82-i86 [ doi:10.1107/S0108270108023457 ]

Sr, Ba and Cd arsenates with the apatite-type structure

T. Ðordevic, S. Sutovic, J. Stojanovic and Lj. Karanovic

Abstract: X-ray diffraction analysis of single crystals of three new arsenates adopting apatite-type structures yielded formula Sr₅(AsO₄)₃F for strontium arsenate fluoride, (I), (Sr_1.66Ba_0.34)(Ba_2.61Sr_0.39)(AsO₄)₃Cl for strontium barium arsenate chloride, (II), and Cd₅(AsO₄)₃Cl_0.58(OH)_0.42 for cadmium arsenate hydroxide chloride, (III). All three structures are built up of isolated slightly distorted AsO₄ tetrahedra that are bridged by Sr²⁺ in (I), by Sr²⁺/Ba²⁺ in (II) and by Cd²⁺ in (III). Compounds (I) and (II) represent typical fluorapatites and chlorapatites, respectively, with F^- at the 2a (0, 0, ${1 \over 4}$ ) site and Cl^- at the 2b (0, 0, 0) site of P6₃/m. In contrast, in (III), due to the requirement that the smaller Cd²⁺ cation is positioned closer to the channel Cl^- anion (partially substituted by OH^-), the anion occupies the unusual 2a (0, 0, ${1 \over 4}$ ) site. Therefore, Cl^- is similar to F^- in (I), coordinated by three A2 cations, unlike the octahedrally coordinated Cl^- in (II) and other ordinary chlorapatites. Furthermore, in (III), using FT-IR studies, we have inferred the existence of H⁺ outside the channel in oxyhydroxyapatites and provided possible atomic coordinates for a H atom in HAsO₄^2-, leading to a proposed formulation of the compound as Cd₅(AsO₄)_3-x(HAsO₄)_xCl_0.58(OH)_0.42-x-(y/2)O_x+(y/2) [white square] _y/2.

Formula: Sr₅(AsO₄)₃F, (Sr_1.66Ba_0.34)(Ba_2.61Sr_0.39)(AsO₄)₃Cl and Cd₅(AsO₄)₃Cl_10.58(OH)_0.42


	Structure factor file (CIF format) (26.9 kbytes) [ doi:10.1107/S0108270108023457/sq3149Isup2.hkl ] Contains datablock I

	Structure factor file (CIF format) (22.6 kbytes) [ doi:10.1107/S0108270108023457/sq3149IIsup3.hkl ] Contains datablock II

	Structure factor file (CIF format) (26.0 kbytes) [ doi:10.1107/S0108270108023457/sq3149IIIsup4.hkl ] Contains datablock III

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